1. Field of the Invention
The present invention relates to a dispersion stabilizer for suspension polymerization of a vinyl compound. More specifically, the invention relates to a dispersion stabilizer for suspension polymerization of a vinyl compound which, by the use of a small amount, exhibits quite an excellent suspension polymerization stability upon providing an effect that polymer scales are less adhered to an inner wall of a polymerization vessel and can produce, through suspension polymerization of a vinyl compound, vinyl polymer granules having a high plasticizer absorption, a sharp particle size distribution, a porosity and a high bulk density.
2. Discussion of the Background
The production of a vinyl polymer such as a vinyl chloride resin has been widely conducted industrially by suspension polymerization in which a vinyl compound such as vinyl chloride is dispersed in an aqueous medium in the presence of a dispersion stabilizer and polymerization is conducted using an oil-soluble initiator. In general, the factors governing the quality of a vinyl polymer include a conversion, a water/monomer ratio, a polymerization temperature, a type and an amount of an initiator, a type of a polymerization vessel, a stirring rate and a type of a dispersion stabilizer. Among these factors, a type of a dispersion stabilizer has a great effect.
The performances required of a dispersion stabilizer for suspension polymerization of a vinyl compound are as follows. (1) It functions, by the use of a small amount thereof, to exhibit quite an excellent suspension polymerization stability and to make the particle size distribution of vinyl polymer granules obtained as sharp as possible, (2) it functions to make polymer granules as uniform and porous as possible to increase a rate of plasticizer absorption for increasing a processability, to facilitate the removal of a monomer such as vinyl chloride remaining in polymer granules and to prevent the formation of fish eyes in a molded product, (3) it functions to form polymer granules having a high bulk density, and (4) scales are not adhered to a polymerization vessel.
As a dispersion stabilizer for suspension polymerization of a vinyl compound, cellulose derivatives such as methylcellulose and carboxymethylcellulose and partially saponified polyvinyl alcohol have been so far used either singly or in appropriate combination. The ordinary dispersion stabilizer, however, do not necessarily meet the foregoing performances satisfactorily.
Further, Japanese Patent Laid-Open Nos. 127,490/1979, 95,104/1989, 140,303/1991, 80,709/1994 and 259,609/1996 propose a dispersion stabilizer for suspension polymerization of a vinyl compound made of a modified polyvinyl alcohol containing an ethylene unit. A dispersion stabilizer made of an ethylene-modified polyvinyl alcohol as proposed in Japanese Patent Laid-Open No. 127,490/1979 is problematic in that the use of a modified polyvinyl alcohol having a high content of an ethylene unit decreases a workability owing to a poor water solubility of a dispersion stabilizer or requires the use of a water-organic solvent medium for improving the solubility of the dispersion stabilizer and the use of the organic solvent invites a factor that waste water after suspension polymerization exerts an adverse effect on the environment. A dispersion stabilizer made of an ethylene-modified polyvinyl alcohol having an ionic group in a side chain as proposed in Japanese Patent Laid-Open Nos. 95,104/1989 and 140,303/1991 is problematic in that a water solubility is improved, but bubbling is liable to occur in the suspension polymerization of vinyl chloride which leads to adhesion of scales to a polymerization vessel. A dispersion stabilizer made of an ethylene-modified polyvinyl alcohol as proposed in Japanese Patent Laid-Open No. 80,709/1994 gives a relatively well-balanced vinyl chloride resin, but a plasticizer absorption or a sharpness of a particle size distribution of the vinyl chloride resin does not necessarily reach a satisfactory level. Further, a dispersion stabilizer using a combination of an ethylene-modified polyvinyl alcohol and a polyvinyl alcohol polymer as proposed in Japanese Patent Laid-Open No. 259,609/1996 improves, because of an excellent anti-foaming effect in a polymerization vessel, a problem that scales are adhered to a reaction vessel by bubbling in the suspension polymerization of vinyl chloride. However, a plasticizer absorption or a sharpness of a particle size distribution of the resulting vinyl chloride resin does not necessarily reach a satisfactory level.
Under these circumstances, the invention aims to provide a dispersion stabilizer for suspension polymerization of a vinyl compound which, by the use of a small amount, exhibits quite an excellent suspension polymerization stability and can produce, through suspension polymerization of a vinyl compound, vinyl polymer granules having a high plasticizer absorption, a sharp particle size distribution, a porosity and a high bulk density. Further, it aims to provide a dispersion stabilizer for suspension polymerization of a vinyl compound which can produce vinyl polymer granules having characteristics that a vinyl compound remaining in vinyl polymer granules is easy to remove, scattering of a powder less occurs in handling and a biting property into a molding machine is good.
The present inventors have assiduously conducted investigations, and have consequently found that the foregoing aims are attained by a dispersion stabilizer for suspension polymerization of a vinyl compound comprising a vinyl alcohol polymer (A) having a content of an ethylene unit of 0.5 to 10 mol %, a degree of saponification of 60 mol % or more and a degree of polymerization of 600 or more and a vinyl alcohol polymer (B) selected from a vinyl alcohol polymer (B1) having a content of an ethylene unit of 0.5 to 10 mol %, a degree of saponification of 20 to 80 mol % and a degree of polymerization of 100 to 600, a vinyl alcohol polymer (B2) having a degree of saponification of 20 to 60 mol % and a degree of polymerization of 100 to 600 and a vinyl alcohol polymer (B3) having a content of an ethylene unit of 0.5 to 10 mol %, a degree of saponification of 90 mol % or more and a degree of polymerization of 1,000 or more. This finding has led to the completion of the invention.
The invention is described in detail below.
The content of the ethylene unit of the vinyl alcohol polymer (A) used in the invention is 0.5 to 10 mol %, preferably 1 to 10 mol %, more preferably 1.5 to 8 mol %. When the content of the ethylene unit is less than 0.5 mol %, adhesion of scales to a reaction vessel is increased in the suspension polymerization of the vinyl compound. When it exceeds 10 mol %, a water solubility of the vinyl alcohol polymer is decreased to worsen a handleability.
The degree of saponification of the vinyl alcohol polymer (A) is 60 mol % or more, preferably 65 to 95 mol %, more preferably 70 to 90 mol %. When the degree of saponification is less than 60 mol %, the water solubility of the vinyl alcohol polymer is decreased to worsen the handleability.
The degree of polymerization of the vinyl alcohol polymer (A) is 600 or more, preferably 600 to 8,000, more preferably 650 to 3,500. When the degree of polymerization of the vinyl alcohol polymer is less than 600, the polymerization stability is decreased in the suspension polymerization of the vinyl compound.
The content of the ethylene unit of the vinyl alcohol polymer (B1) used in the invention is 0.5 to 10 mol %, preferably 1 to 10 mol %, more preferably 1.5 to 8 mol %. When the content of the ethylene unit is less than 0.5 mol %, adhesion of scales to a reaction vessel is increased in the suspension polymerization of the vinyl compound. When it exceeds 10 mol %, the water solubility of the vinyl alcohol polymer is decreased to worsen the handleability.
The degree of saponification of the vinyl alcohol polymer (B1) is 20 to 80 mol %, preferably 25 to 80 mol %, more preferably 30 to 75 mol %. When the degree of saponification deviates from the range of 20 to 80 mol %, the particle size distribution of vinyl polymer granules obtained by the suspension polymerization of a vinyl compound is widened.
The degree of polymerization of the vinyl alcohol polymer (B1) is 100 to 600, preferably 150 to 550, more preferably 200 to 550. When the degree of polymerization is less than 100, the polymerization stability is decreased in the suspension polymerization of the vinyl compound. When it exceeds 600, the plasticizer absorption of the vinyl polymer obtained by the suspension polymerization of the vinyl compound is decreased.
Further, it is preferable that a difference in degree of saponification between the vinyl alcohol polymer (A) and the vinyl alcohol polymer (B 1) is 15 mol % or more and/or a difference in degree of polymerization therebetween is 200 or more.
The degree of saponification of the vinyl alcohol polymer (B2) used in the invention is 20 to 60 mol %, preferably 25 to 60 mol %, more preferably 30 to 55 mol %. When the degree of saponification deviates from the range of 20 to 60 mol %, the particle size distribution of the vinyl polymer granules obtained by the suspension polymerization of the vinyl compound is widened.
The degree of polymerization of the vinyl alcohol polymer (B2) is 100 to 600, preferably 150 to 550, more preferably 200 to 550. When the degree of polymerization is less than 100, the polymerization stability is decreased in the suspension polymerization of the vinyl compound. When it exceeds 600, the plasticizer absorption of the vinyl polymer obtained by the suspension polymerization of the vinyl compound is decreased.
Further, it is preferable that a difference in degree of saponification between the vinyl alcohol polymer (A) and the vinyl alcohol polymer (B2) is 15 mol % or more and/or a difference in degree of polymerization therebetween is 200 or more.
The content of the ethylene unit of the vinyl alcohol polymer (B3) used in the invention is 0.5 to 10 mol %, preferably 1 to 10 mol %, more preferably 1.5 to 8 mol %. When the content of the ethylene unit is less than 0.5 mol %, adhesion of scales to a reaction vessel is increased in the suspension polymerization of the vinyl compound. When it exceeds 10 mol %, the water solubility of the vinyl alcohol polymer is decreased to worsen the handleability.
The degree of saponification of the vinyl alcohol polymer (B3) is 90 mol % or more, preferably 91 mol % or more, more preferably 92 mol % or more. When the degree of saponification is less than 90 mol %, a resin having a high bulk density cannot be obtained.
The degree of polymerization of the vinyl alcohol polymer (B3) is 1,000 or more, preferably 1,000 to 8,000, more preferably 1,000 to 3,500. When the degree of polymerization is less than 1,000, a resin having a high bulk density cannot be obtained.
Further, it is preferable that a difference in degree of saponification between the vinyl alcohol polymer (A) and the vinyl alcohol polymer (B3) is 5 mol % or more and/or a difference in degree of polymerization therebetween is 200 or more. It is more preferable that the difference in degree of saponification is 10 mol % or more and/or the difference in degree of polymerization is 200 or more.
In the dispersion stabilizer for suspension polymerization in the invention, a vinyl alcohol polymer (A)/vinyl alcohol polymer (B) ratio is not strictly limited. A component (A)/component (B) weight ratio is usually 95/5 to 20/80.
When the vinyl alcohol polymer (B) is the vinyl alcohol polymer (B1) or the vinyl alcohol polymer (B2), the component (A)/component (B) weight ratio is preferably 95/5 to 30/70, more preferably 95/5 to 50/50. When the component (A)/component (B) weight ratio exceeds 95/5, there is a tendency that the plasticizer absorption of the vinyl polymer obtained by the suspension polymerization of the vinyl compound is decreased or the particle size distribution is widened. When it is less than 20/80, the polymerization stability might be decreased in the suspension polymerization of the vinyl compound.
When the vinyl alcohol polymer (B) is the vinyl alcohol polymer (B3), the component (A)/component (B) weight ratio is preferably 80/20 to 30/70, more preferably 70/30 to 40/60. When the component (A)/component (B)weight ratio exceeds 95/5, there is a likelihood that a resin having a high bulk density is not obtained. When it is less than 20/80, the plasticizer absorption tends to be decreased.
In the invention, the amount of the dispersion stabilizer for suspension polymerization is not particularly limited. It is preferably 0.01 to 5 parts by weight, more preferably 0.02 to 2 parts by weight, further preferably 0.02 to 1 part by weight per 100 parts by weight of the vinyl compound. When it is less than 0.01 part by weight, the polymerization stability tends to be decreased in the suspension polymerization of the vinyl compound. When it exceeds 5 parts by weight, there is a tendency that a waste liquor after the suspension polymerization is opaque and chemical oxygen demand (COD) is increased.
The vinyl alcohol polymer (B1) and the vinyl alcohol polymer (B2) used in the invention are insoluble in water or dispersible in water. A self-emulsifiability can be imparted by introducing therein an ionic group such as a sulfonic group, an amino group, an ammonium group, a carboxyl group or a cationic group. Unless an ionic group is introduced, the water solubility or the water dispersibility of the vinyl alcohol polymer (B1) and the vinyl alcohol polymer (B2) might be decreased to worsen the handleability.
Further, in the vinyl alcohol polymer (A) and the vinyl alcohol polymer (B3) used in the invention, the water solubility can be increased by introducing an ionic group such as an ammonium group, a carboxyl group, a sulfonic group or an amino group. Alternatively, a nonionic group or a (long-chain) alkyl group may be introduced. It is advisable that the vinyl alcohol polymer (A) and the vinyl alcohol polymer (B3) is soluble in water having a temperature of 5 to 100xc2x0 C., preferably 10 to 90xc2x0 C.
The degree of saponification of the vinyl alcohol polymer having introduced therein an ionic group, a nonionic group or a (long-chain) alkyl group is obtained from a ratio of a vinyl ester group and a vinyl alcohol group, and a degree of saponification of an ionic group, a nonionic group or a (long-chain) alkyl group introduced is not included therein.
In the invention, a method for production of the vinyl alcohol polymer (A), the vinyl alcohol polymer (B1) and the vinyl alcohol polymer (B3) is not particularly limited. They can be obtained by a known method, for example, a method described in Japanese Patent Laid-Open No. 259,609/1996, namely, a vinyl ester monomer, ethylene and as required, a monomer having an ionic group are copolymerized and the resulting copolymer is saponified in a usual manner, or an end modification method in which a vinyl ester monomer and ethylene are copolymerized in the presence of a thiol compound such as a thiolacetic acid or mercaptopropionic acid and the copolymer is saponified.
Further, a method for production of the vinyl alcohol polymer (B2) is not particularly limited. It can be obtained by a known method, namely, a vinyl ester monomer and as required, a monomer having an ionic group are copolymerized and the resulting copolymer is saponified in a usual manner, or an end modification method in which a vinyl ester monomer is polymerized in the presence of a thiol compound such as a thiolacetic acid or mercaptopropionic acid and the polymer is saponified.
As a method for copolymerizing a vinyl ester monomer with ethylene, a known method such as a solution polymerization method, a bulk polymerization method, a suspension polymerization method or an emulsion polymerization method can be employed. As a polymerization initiator, an azo initiator, a peroxide initiator or a redox initiator is selected, as required, according to a polymerization method. As the saponification reaction, alcoholysis or hydrolysis using a known alkali catalyst or acid catalyst can be employed. Among others, a saponification reaction using methanol as a solvent and an NaOH catalyst is simple and most preferable.
Examples of the vinyl ester monomer herein include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl versatate, vinyl caproate, vinyl caprylate, vinyl laurylate, vinyl palmitate, vinyl stearate, vinyl oleate and vinyl benzoate. Of these, vinyl acetate is most preferable.
In the invention, the monomer having the ionic group which is used to introduce the ionic group into the vinyl alcohol polymer as required is not particularly limited. Examples thereof include sulfonic acid-containing monomers such as ethylenesulfonic acid, (meth)allylsulfonic acid, sulfoalkyl maleate, sulfoalkyl (meth)acrylate and acrylamido-2-methylpropanesulfonic acid and salts thereof; amino or ammonium group-containing monomers such as N(1,1-dimethyl-3-dimethylaminopropyl)(meth)acrylamide, N(1,1-dimethyl-3-dimethylaminobutyl)(meth)acrylamide, N-vinylimidazole, 2-methyl-N-vinylimidazole, vinyl-3-dimethylaminopropyl ether, vinyl-2-dimethylaminoethyl ether, allyl-3-dimethylaminopropyl ether, allyldimethylamine and methallyldimethylamine; and carboxyl group-containing monomers such as crotonic acid, maleic acid, fumaric acid, itaconic acid and (meth)acrylic acid.
In the invention, the vinyl alcohol polymer (A) and the vinyl alcohol polymer (B) may contain another monomer unit unless the gist of the invention is impaired. Examples of the available comonomer include xcex1-olefins such as propylene, n-butene and isobutylene; acrylic acid and salts thereof; acrylic acid esters such xe2x80x98as methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate and octadecyl acrylate; methacrylic acid and salts thereof; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate and octadecyl methacrylate; acrylamide; acrylamide derivatives such as N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetonacrylamide, acrylamidopropanesulfonic acid and salts thereof, acrylamidopropyldimethylamine and salts thereof or quaternary salts thereof, N-ethylolacrylamide and derivatives thereof; methacrylamide; methacrylamide derivatives such as N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidopropanesulfonic acid and salts thereof, methacrylamidopropyldimethylamine and salts thereof or quaternary salts thereof, N-methylolmethacrylamide and derivatives thereof; vinyl ethers such as methylvinyl ether, ethylvinyl ether, n-propylvinyl ether, i-propylvinyl ether, n-butylvinyl ether, i-butylvinyl ether, t-butylvinyl ether, dodecylvinyl ether and stearylvinyl ether; nitriles such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride and vinyl fluoride; vinylidene halides such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; unsaturated dicarboxylic acids such as maleic acid, itaconic acid and fumaric acid and salts thereof or esters thereof; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate.
In the invention, the vinyl alcohol polymer (A) and the vinyl alcohol polymer (B) are usually dissolved or dispersed in an aqueous medium either separately or by being mixed, and subjected to the suspension polymerization of the vinyl compound.
In the dispersion stabilizer for suspension polymerization in the invention, a product heat-treated is used as the vinyl alcohol polymer (A) to further improve the stability in the suspension polymerization. The conditions for the heat treatment are not particularly limited. However, it is advisable that the heat treatment is conducted in an atmosphere of oxygen, air or nitrogen at a temperature of 100 to 200xc2x0 C. for 0.5 to 20 hours. When the temperature of the heat treatment is less than 100xc2x0 C., the effect of improving the stability in the suspension polymerization by the heat treatment is, in some cases, not satisfactorily exhibited. When it exceeds 200xc2x0 C., the dispersion stabilizer for suspension polymerization might be colored.
At this time, for controlling the coloration of the dispersion stabilizer for suspension polymerization by the heat treatment, it is advisable that the vinyl alcohol polymer (A) is washed with a solvent such as methanol before the heat treatment.
When the heat-treated product is used as the vinyl alcohol polymer (A) in the dispersion stabilizer for suspension polymerization in the invention, it is advisable that the vinyl alcohol polymer (A) contains an acid having pKa at 25xc2x0 C. of 3.5 to 5.5 and/or its metal salt (C), therefore the suspension polymerization stability is markedly excellent by the use of a small amount, and the coloration can be controlled in processing the polymer obtained by the suspension polymerization. The type of the available acid is not particularly limited. Specific examples thereof include acetic acid (pKa 4.76), propionic acid (pKa 4.87),butyric acid (pKa 4.63), octanoic acid (pKa 4.89), adipic acid (pKa 5.03), benzoic acid (pKa 4.00), formic acid (pKa 3.55), valeric acid (pKa 4.63), heptoic acid (pKa 4.66), lactic acid (pKa 3.66), phenylacetic acid (pKa 4.10), isobutyric acid (pKa 4.63) and cyclohexanecarboxylic acid (pKa 4.70). Acids that can be used especially preferably in view of the effect provided are acetic acid, propionic acid and lactic acid.
Further, the metal salt of the acid is not particularly limited. Usually, metal salts of the foregoing acids and alkali metals such as sodium and potassium are used. Sodium acetate is especially preferable.
The content of the acid and/or its metal salt (C) is preferably 0.05 to 2 parts by weight, more preferably 0.1 to 1.7 parts by weight, further preferably 0.2 to 1.5 parts by weight per 100 parts by weight of the vinyl alcohol polymer (A). When the content of the acid and/or its metal salt (C) is less than 0.05 part by weight based on the vinyl alcohol polymer (A), the effect of improving the stability in the suspension polymerization by the heat treatment is decreased. When it exceeds 2 parts by weight, the dispersion stabilizer for suspension polymerization is colored in the heat treatment or the polymer is colored in processing the polymer obtained by the suspension polymerization. Thus, it is undesirous.
The method for suspension polymerization of the vinyl compound using the dispersion stabilizer for suspension polymerization in the invention is described below.
In the suspension polymerization of the vinyl compound in the aqueous medium using the dispersion stabilizer for suspension polymerization in the invention, the temperature of the aqueous medium is not particularly limited. Cold water of approximately 20xc2x0 C. and hot water of 90xc2x0 C. or more can preferably be used. This aqueous medium can be pure water or an aqueous medium made of an aqueous solution containing pure water and various additives or an aqueous medium containing another organic solvent. When the aqueous medium is charged into a polymerization reaction system, the amount thereof may be an amount capable of satisfactorily heating the polymerization reaction system. Further, in order to increase a heat removal efficiency, a polymerization vessel fitted with a reflux condenser is preferably used.
The dispersion stabilizer for suspension polymerization in the invention may be used either singly or in combination with water-soluble polymer such as polyvinyl alcohol, gelatin and water-soluble cellulose ethers represented by methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose and hydroxypropylmethyl cellulose; oil-soluble emulsifying agents such as sorbitan monolaurate, sorbitan trioleate, glycerin tristearate and an ethylene oxide/propylene oxide block copolymer; and water-soluble emulsifying agents such as polyoxyethylenesorbitan monolaurate, polyoxyethyleneglycerin oleate and sodium laurate; which are commonly used in the suspension polymerization of a vinyl compound in an aqueous medium. The amounts thereof are not particularly limited, and are preferably 0.01 to 1.0 part by weight per 100 parts by weight of the vinyl compound.
The other additives can also be added as required. Examples of the additives include agents for controlling degree of polymerization such as acetaldehyde, butylaldehyde, trichloroethylene, perchloroethylene and mercaptans; and polymerization inhibitors such as a phenolic compound, a sulfur compound and an N-oxide compound. Further, pH adjustors, scale preventing agents and crosslinking agents can also be added as required, and the additives may be used in combination. Meanwhile, polymerization initiators which have been so far used in the polymerization of a vinyl compound such as vinyl chloride can be used. Examples thereof can include percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate and diethoxyethyl peroxydicarbonate; per-ester compounds such as t-butyl peroxyneodecanate, a-cumyl peroxyneodecanate and t-butyl peroxydecanate; peroxides such as acetylcyclohexylsulfonyl peroxide and 2,4,4-trimethylpentyl 2-peroxyphenoxyacetate; and azo compounds such as 2,2xe2x80x2-azobisisobutyronitrile, 2,2xe2x80x2-azobis(2,4-dimethylvaleronitrile) and 2,2xe2x80x2-azobis(4-methoxy-2,4-dimethylvaleronitrile). Further, these can be used in combination with potassium persulfate, ammonium persulfate and hydrogen peroxide.
Examples of the vinyl compound to which the dispersion stabilizer for suspension polymerization in the invention can be applied include vinyl chloride, vinylidene chloride, alkylvinyl ether, maleic anhydride, acrylonitrile, itaconic acid, styrene, vinyl esters such as vinyl acetate and vinyl propionate, (meth)acrylic acid esters such as methyl (meth)acrylate and ethyl (meth)acrylate, and xcex1-olefins such as ethylene, propylene, isobutene and isoprene. Typical of these vinyl compounds is vinyl chloride, and vinyl chloride can be used either singly or in combination with another monomer in which vinyl chloride is a main component (vinyl chloride 50% by weight or more). Examples of the comonomer to be copolymerized with vinyl chloride include vinyl esters such as vinyl acetate and vinyl propionate, (meth)acrylic acid esters such as methyl (meth) acrylate and ethyl (meth) acrylate, xcex1-olefins such as ethylene and propylene, maleic anhydride, acrylonitrile, itaconic acid, styrene, vinylidene chloride and vinyl ether.
In the suspension polymerization of the vinyl compound using the dispersion stabilizer for suspension polymerization in the invention, the proportions of the components, the polymerization temperature and the like can be determined according to the conditions ordinarily employed in the suspension polymerization of the vinyl compound such as vinyl chloride. Further, the order of charging the vinyl compound, the polymerization initiator, the dispersion stabilizer, the aqueous medium and other additives and the proportions thereof are not particularly limited at all. Still further, a method in which hot water is used as the aqueous medium and the vinyl compound is heated before being charged into a polymerization vessel is preferable.